Multi-photon quantum interference from multiple photon emitters is considered to be a key ingredient for certain quantum processing applications. To achieve such interference requires the photons from different emitters to be quantum mechanically indistinguishable.
Multi-photon quantum interference from multiple gas state emitters, including emission from a single atom/ion in a trap, is known. This is achieved by generating photon emission from gas state emitters which is identical in terms of bandwidth, frequency, and polarization such that photons from different emitters are quantum mechanical indistinguishable. These identical photons can then be overlapped in a beamsplitter to achieve quantum interference.
The aforementioned approach is problematic for solid state emitters. This is because the energies of optical transitions in solid state systems vary due to different strain environments. Differences in emission characteristics of solid state emitters may be caused by impurities and/or intrinsic crystal defects such as dislocations. As such, photons emitted from two different solid state emitters vary in terms of bandwidth, frequency, and polarization and are quantum mechanically distinguishable. Accordingly, such photons do not undergo quantum interference when overlapped on a beamsplitter or comparable arrangement.
One solution to the aforementioned problem is to reduce the resolution of a detector arrangement used to detect photons from photon emitters to an extent that the photons from different sources are indistinguishable to the detectors. For example, by using detectors with a high time resolution this in turn leads to a low frequency resolution which can render the photons indistinguishable. However, single photon emission from defects in solid state materials can be very weak. For example, the photon emissive nitrogen-vacancy defect (NV−) in diamond material, which is a leading candidate for solid state quantum processing applications, exhibits a broad spectral emission associated with a Debye-Waller factor of the order of 0.05, even at low temperature. Emission of single photons in the Zero-Phonon Line (ZPL) is then extremely weak, typically of the order of a hundred thousand of photons per second. Furthermore, due to emission losses only approximately 1% of this emission is typically detected resulting in low count rates. Such count rates are insufficient for the realization of advanced quantum information processing protocols within reasonable data acquisition times based on photon interference using high time resolution. (i.e. low frequency resolution) detectors.
In fact, the aforementioned issues are so problematic that multi-photon quantum interference from multiple solid state quantum registers has not been demonstrated in practice to date. In this regard, it should be noted that a solid state quantum register comprises both nuclear and electron spins coupled together. An electron spin can function as a control qubit with optical spin state detection and fast high fidelity coherent manipulation. A nuclear spin can function as a memory qubit which has weak interaction with the surrounding environment. Together, an electron spin and a nuclear spin coupled can form a quantum register. An example of such a quantum register is a nitrogen-vacancy defect in diamond material which has resolvable electron spin states which are optically addressable and coupled to nuclear spin states of the nitrogen nucleus and/or 13C nuclei in the surrounding diamond lattice. It should be noted that a quantum register of this kind differs from systems which only comprise single spin emitters, decoupled spin states, or emitters which do not comprise spin states which can be resolved to function as a quantum register.
In relation to the above, it is noted that multi-photon interference has been observed from more simple solid state systems such as quantum dots, single molecules adsorbed onto a surface, and F-dopants in ZnSe [see, for example, R. Lettow et al., Physical Review Letters 104 (2010); Patel et al., Nature Photonics 2010, DOI: 10.1038/NPHOTON.2010.161; Sanaka et al., PRL 103, 053601 (2009); and Flagg et al., Phys. Rev. Lett. 104, 137401 (2010)]. However, multi-photon quantum interference from multiple spin resolved solid state quantum registers has not been demonstrated in practice to date due to the previously described problems. Quantum dots in III-V semiconductors relate only to electronic transitions and thus are unsuitable for quantum information processing applications requiring a nuclear spin memory qubit. Single molecules adsorbed onto a surface do not exhibit spin resolved emission suitable for information processing. Furthermore, surface crystallized single molecule systems are inherently fragile systems which may be unsuitable for commercial device applications. F-dopants in ZnSe have not been demonstrated to comprise spin resolved solid state quantum registers in which resolvable electron spin states are coupled to one or more nuclear spins.
Accordingly, there is a need to provide a device which is capable of providing multi-photon quantum interference from multiple solid state quantum registers, particularly in reasonable data acquisition times.